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  Trailer: Tongue Weight v. Jack Load

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Author Topic:   Trailer: Tongue Weight v. Jack Load
jimh posted 08-12-2012 10:58 PM ET (US)   Profile for jimh   Send Email to jimh  
The tongue of my trailer is located 18-feet from the front axle. The trailer jack is located 42-inches behind the tongue. For each 1-lbs of weight on the tongue, how much load will bear on the jack when the trailer is supported by the jack instead of the tongue?

My rough estimation is a follows:

18-feet x 12-inches/1-foot = 216-inches = axle to tongue distance
216-inches - 42-inches = 174 =axle to jack distance

Load on jack per 1-lbs-load on tongue = 1.24-lbs

If my tongue weight is estimated to be 350-lbs, then the load on the jack will be about 434.4-lbs.

Chuck Tribolet posted 08-12-2012 11:44 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I concur.

Chuck

L H G posted 08-12-2012 11:45 PM ET (US)     Profile for L H G    
No, Jim. The load on the jack will always be less than the tongue weight at the ball, which is the maximum moment arm. Having a jack behind the hitch ball is the same as moving the trailer wheels forward, which we all know will lighten tongue load.
gusgus posted 08-13-2012 02:38 AM ET (US)     Profile for gusgus  Send Email to gusgus     
LHG. I am pretty sure you aren't correct.
Moment arm is a mathematical measurement, used to calculate center of gravity. It isn't necessarily the measurement of weight.

Consider the tongue jack a foot forward of the axle. Will the weight be less or more? MORE, lots more.

The calculation Jim has is within reason, without me spending an inordinate amount of time doing the math.

But simple logic says length (or moment arm increase) reduces required force to support a given weight. Decrease the distance of the support from the center of gravity and the weight shall increase up to the total weight of the supported item (at the balance point). So Jim is very likely on the right track.

jimh posted 08-13-2012 06:59 AM ET (US)     Profile for jimh  Send Email to jimh     
Larry--the trailer tongue is a lever that is lifting the weight of the boat. The longer the lever, the less force needed to lift the weight.
contender posted 08-13-2012 07:16 AM ET (US)     Profile for contender  Send Email to contender     
[Changes topic. Please start a new thread for that topic. Thank you.--jimh]
newt posted 08-13-2012 08:33 AM ET (US)     Profile for newt  Send Email to newt     
The analysis above assumes that all the weight is on the front axle. If you have a dual axle trailer, then the pivot point moves back a bit, which would make the ratio less than the 1.24 calculated by some amount.

With conventional leaf spring suspension, I would surmise that the pivot point is very close to the center of the two axles. With torsion axles, the equation is much more complicated.

Chuck Tribolet posted 08-13-2012 08:39 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I missed that it was a dual axle trailer, so the calculation
is not correct, and will likely be a function of how high
the jack is cranked.

Chuck

jimh posted 08-13-2012 11:11 AM ET (US)     Profile for jimh  Send Email to jimh     
My particular trailer is a tandem axle trailer. For the case where the jack holds the trailer at the same height that the ball hitch held the tongue, I do not see any reason that the presence of tandem axles would affect the calculation.

If you begin to raise the trailer tongue with the jack to a greater height, you will tend to unload weight from the forward axle, and that weight will bear on the jack. Of course, this also happens when hitched in the case where there is uneven terrain and the trailer weight distribution shifts.

As I said, my calculations were intended "for a rough estimate," and I am satisfied that they are sufficiently accurate for my purpose.

My purpose, by the way, is to estimate the trailer load on the tongue jack so that I can appropriately size the jack. The old jack failed. The failure occurred while the jack was stowed and not bearing any weight, but I suspect that the real cause of the failure was from overloading of the jack with too much weight. The overloading occurred when the coupler on the trailer became reluctant to disengage and release the ball hitch. The trailer started lifting the rear end of the truck. I have since lubricated the locking mechanism and developed a sensitivity to the trailer coupler getting stuck. In another thread I will discuss the coupler situation in more detail.

I replaced the old jack. It was a Fulton and rated for only 750-lbs. The new jack is also a Fulton but is rated for 1,500-lbs.

Tom W Clark posted 08-13-2012 11:27 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Chuck is correct. The calculations are based on the simple principles of the lever. A single axle trailer is a lever where the fulcrum is the axle.

With a dual axle trailer, however, the fulcrum is between the two axles. That distance can be significant and it will affect the calculations.

Keeper posted 08-13-2012 12:55 PM ET (US)     Profile for Keeper  Send Email to Keeper     
And the point of this post is what ??????

How about a tip rather than some mathematical nonsense.

Here's one.

I use a wood timber (4"X6") when I store my boat for longer periods of time.
I raise the tongue using the jack to the desired height, then prop the wood block in a vertical fashion from the ground to a point under the trailer where the side beams and tongue come together.
I then lower the jack to release the weight or pressure from the jack, to the wood.
I leave the jack wheel still lightly touching the ground so if there is movement, or the block some how becomes dislodged it will take up the slack.

This has helped my jacks last MUCH longer and operate flawlessly for a longer period of time.

There . . . something useful.

This sight is great, but sometimes I feel it 'reaches' for content.

[Please note that this is a discussion of trailer tongue and jack loads, and we are not discussing the discussion itself. When readers want to change the topic of the discussion from its actual topic to begin a discussion of the discussion itself or to the website as a whole, I have to point out that the website has very little interest in collecting and organizing information about its discussions themselves or the website itself. The website prefers to limit its discussions to boating topics, and to not wander off-topic to begin discussions about the discussions themselves or the website itself. Please try to limit your comments to the boating topics and avoid the tendency to enter into a discussion only to discuss the participants or the discussion itself or the website. These sidebars to not add much boating information and are generally only distractions to the boating topic--jimh]

newt posted 08-13-2012 01:11 PM ET (US)     Profile for newt  Send Email to newt     
quote:
How about a tip rather than some mathematical nonsense.

It's only nonsense if you are not smart enough to understand it.

L H G posted 08-13-2012 01:12 PM ET (US)     Profile for L H G    
Most weight is on the tandem wheels, the rest is on the tongue. Jim is saying that by moving the jack back, he is taking more weight off the trailer wheels and adding it to the tongue. That, I suppose could be correct, but I don't know why anybody would want to do that. That was not the assumption as far as I can tell. If the jack was exactly at the center of gravity, the whole rig would theoretically balance (teeter) on the jack carrying 100% of the load, with the trailer wheels off the ground. If moving the jack back unloads the trailer wheels a little, then Jim is correct.

gnr posted 08-13-2012 01:15 PM ET (US)     Profile for gnr    
What kind of wood do you use? Have you researched the structural characteristics of various would types to ensure you piece of wood is suitable for the task?

What about climate and the effect it may have on the piece of woods stability over time. Have you calculated the degree of warpage that could be endured before a catastrophic failure?

How about the conditions on the trailer jack from leaving the wheel in a sort of suspended condition where it is neither fully loaded on the ground or in a horizontal position, the condition in which it is typically placed in.

Come on man. Get your act together!

LOL

newt posted 08-13-2012 01:16 PM ET (US)     Profile for newt  Send Email to newt     
Jim, it sounds like you have ample jack capacity now, and will not need to use a piece of wood as is suggested above.

On a side note, my dual axle trailer springs meet in the middle at a pivoting bracket. If the rear axle is loaded more than the front axle, then the bracket rotates so that the forward end of the rear leaf springs are now higher than the rear end of the forward springs, which I presume will compress the front springs a bit more and somewhat equalize the load that each axle takes. Not sure if all trailers are set up this way or just my former commercial products division "launch out of a jungle" trailer.

Jerry Townsend posted 08-13-2012 01:18 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
If the angle of the boat is the same - on the hitch or jack/stand, - Jimh's calculation - 350 X 216 / 174 - is close enough . Well it could be 350 X (216 + 25) / (174 + 25) - or some other increment - which would give 423 - a difference of less than 5%.

When dealing with multiple axles, the calculation gets real messy! - as the solution is not simple as above - but is implicit ---.

My suggestion when dealing with multiple axles is just to take the mid-point of all axles as the "effective" pivot point. ---- Jerry/Idaho

-

Keeper posted 08-13-2012 01:27 PM ET (US)     Profile for Keeper  Send Email to Keeper     
Computer Boaters, pfff.
Whalrman posted 08-13-2012 01:46 PM ET (US)     Profile for Whalrman  Send Email to Whalrman     
When a question of concern over tongue load or trailer jack stand load arrises I use a "Load Master" trailer tongue scale. Neat little devise that I got from my father when he used to go RV'ing alot and aided in propper loading for their Airstream trailer for travel. Three models to choose from: 0-1000#, 0-2000#, 0-5000#. Takes the guess work out it. No web address as it was made/packaged before such,probbly google it though.
Whalrman posted 08-13-2012 02:14 PM ET (US)     Profile for Whalrman  Send Email to Whalrman     
www.sherline.com, McMaster-Carr is a distributor for the Load Master scales.
gusgus posted 08-13-2012 02:40 PM ET (US)     Profile for gusgus  Send Email to gusgus     
The discussion is a large part silly.

[Please note that this is a discussion of trailer tongue and jack loads, and we are not discussing the discussion itself. When readers want to change the topic of the discussion from its actual topic to begin a discussion of the discussion itself, I have to point out that the website has very little interest in collecting and organizing information about its discussions, that is the discussion themselves. The website prefers to limit its discussions to boating topics, and to not wander off-topic to begin discussions about the discussions themselves. Please try to limit your comments to the boating topics and avoid the tendency to enter into a discussion only to discuss the participants or the discussion itself. These sidebars to not add much boating information and are generally only distractions to the boating topic--jimh]

Because the information sought about loading and the (tipping point) center of gravity, is easily figured. Not rocket science, but normal CG calculations done by thousands of crews and pilots world wide.

It make absolutely no difference if it is a tandem axle trailer, single axle trailer or the space shuttle, the calculations are the same.

To estimate them, uses the same mathematical calculation as if the weights were known. Those weights would be taken at all tire points and tongue jack or hitch location. A datum point has to have been chosen, so measurements/weights could be synchronized. Simple mathematics are used to find the CG. Once this is done, movements of the tongue jack location would be easy in figuring weight applied.

But it is without question that the CG point is 100% of the load, as the tongue jack moves further from the CG point the load lessens (within reason).

jimh posted 08-13-2012 03:27 PM ET (US)     Profile for jimh  Send Email to jimh     
I am completely happy with my calculations, my statements, and my jack purchase. I am also completely happy with the website and this discussion. Readers who think this is silly should sit on their hands when reading the website.
newt posted 08-13-2012 04:24 PM ET (US)     Profile for newt  Send Email to newt     
Gusgus I for one would be interested to see how you take a very simple calculation such as JimH performed satisfactorily in the very first post of this thread and do it any faster or simpler using hypothetical weights at all the tire points, jack point, and hitch points. It should only take you a minute or two as thousands of pilots and crews worldwide have already performed it. Enlighten us please.
gusgus posted 08-14-2012 03:51 AM ET (US)     Profile for gusgus  Send Email to gusgus     
after 20 seconds of searching this came up.
A clear representation of the calculations required to figure any CG .


Do you want me to do the math for you, because it is simple math. However if you need me to, I will.

http://www.wikihow.com/Calculate-Center-of-Gravity

newt posted 08-14-2012 08:24 AM ET (US)     Profile for newt  Send Email to newt     
Very good google skills. My kids can do that too. I was more interested in you proving how silly this thread is and showing us how you and thousands of aircraft mechanics would have solved the problem so much easier. My mistake, I didn't know you were just going to provide an internet pointer to a formula that is old news on this discussion at this point.
jimh posted 08-14-2012 08:28 AM ET (US)     Profile for jimh  Send Email to jimh     
The method that is explained in the link that the search conducted by GUSGUS found in 20-seconds--which must be one of the longest GOOGLE searches in history--is precisely the method I used in my initial article to find the ratio of the weight bearing on the tongue to the weight that would bear on the jack placed closer to the axle. I did not provide such an elaborate explanation of the method as GUSGUS has. I thought most readers would understand the method, and to give a link to an elaborate explanation like the one GUSGUS has provided would be silly.
Whalrman posted 08-14-2012 09:42 AM ET (US)     Profile for Whalrman  Send Email to Whalrman     
I still like my "Load Master" trailer tongue scale~
Jerry Townsend posted 08-14-2012 12:57 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
AArrrggghh - I just wasted 20 seconds to see where this thread is heading. My suggestion - use your high school physics first.

But then, one of my first questions - why bring in the CG - as it is not important in Jimh's initial question.

Oh - GUSGUS - try putting your datum at the center of the teeter-totter. So much for your google science.

The calculation initially made by Jimh is basically correct. The consideration of multiple axles, as addressed above are appropriate.

Let's make our contributions faotual, objective and constructive - and addressing the initial question/comment and strictly related comments. ---- Jerry/Idaho

PeteB88 posted 08-14-2012 01:13 PM ET (US)     Profile for PeteB88  Send Email to PeteB88     
SO - what have we learned today?????
gusgus posted 08-14-2012 03:01 PM ET (US)     Profile for gusgus  Send Email to gusgus     
Ok fine guys,
I was not addressing Jim's math, but addressing methodology comments on the thread.
I posted a link so I didn't have to attempt to explain my methods for CG and load calcs which I have been using for years. They are tough to explain, but simple to accomplish. If it would add to the discussion, I would gladly explain this. But it sounds like jimh is in the mind set that this is considered counterproductive.

gnr posted 08-14-2012 03:29 PM ET (US)     Profile for gnr    
How I would have done this is as follows:

Damn, I need a new jack.

Hmmmm, looks like they've got different sizes of these things.

Well, I reckon my boat/trailer weighs somewhere a tad south of 3000 lbs.

I further reckon that puts my tongue weight a tad south of 300 lbs. Let's see..... uuummmmpphhhhhhh, oowwwww!!!!. Ayup, that sounds about right.


Now let me see.... If it weighs that there than its gonna weigh a smidgeon more back there. (see Mr Livingston? I told you I weren't sleeping!) Ain't no way its gonna double though. I figured on the high side up to this point so if I double the conservative guestimation on the tongue weight I figure I would be good to go with one of them 500 lb rated capacity units but I'm not always paying attention and I unhook this thing in the middle of the night after a long day on the water I might lift my truck up a notch or too before I notice it and since i'm getting close to the limits of the 500lb capacity jack I reckon I'll order me up a 750 lb jack and call it good.

HONEY!!!!! Where'd you hide the credit card this time?

newt posted 08-14-2012 08:37 PM ET (US)     Profile for newt  Send Email to newt     
Way back when, one of my engineering professors told us that "If civil engineers built airplanes, they would never get off the ground."

I reckon this is because they take the calculated loads and factor them up. Then they take the calculated strength of the building materials and factor it down. The end product is conservatively built and still allows for some construction deficiencies.

There may have been a point to this post, but it took so long to type on my smartphone now I can't remember.

David Pendleton posted 08-14-2012 09:35 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I don't know exactly what my tongue weight is.

I didn't want to worry about it. So, I have a Class V hitch and a ball-mount with a 10,000/1,000 rating. My dual-wheel tongue jack is rated for 1,500 pounds.

Problem solved.

bretm1 posted 08-14-2012 09:50 PM ET (US)     Profile for bretm1  Send Email to bretm1     
Not trying to derail this or change topic.

Tongue is 18 feet from front axle??? Don't like to question the site owner, but are you sure?

18 feet seems really far even if you are driving a huge SUV.

If I missed something delete this.

bretm1 posted 08-14-2012 09:54 PM ET (US)     Profile for bretm1  Send Email to bretm1     
Oops, 18 feet from front trailer axle, I imagine.


delete delete

jimh posted 08-16-2012 11:50 PM ET (US)     Profile for jimh  Send Email to jimh     
Bret--The loads and calculations are independent of the vehicle that will tow the trailer, the arrangements of the towing vehicle's axles, or the type of vehicle. The presumption that the vehicle was an SUV is not implied anywhere in the initial article or by any other comments. You introduced the notion of an SUV yourself. It was not implied in the article.
jimh posted 08-17-2012 12:06 AM ET (US)     Profile for jimh  Send Email to jimh     
Tom says:

"Chuck is correct."

And Chuck says:

"...the calculation is not correct, and will likely be a function of how high the jack is cranked."

Actually Chuck is not quite correct. The influence of the tandem axles will be the same on the tongue weight load as on the jack load. Chuck implies the influence only occurs when the jack is used to raise the trailer tongue. The same increase in loading would also occur on the tongue if a difference in terrain raised the tongue height relative to the axles. I simply calculated the ratio of those loads. As I pointed out, the tandem axle influence is negligible as long as the jack does not raise the trailer any higher than it was when hitched on the tongue. A simple assumption about tandem axle trailers might be to assume the fulcrum of the load bearing was at a midpoint between the trailer axles. This would increase the distance from axle load point to tongue. The result would be the jack load would tend to not increase as much with its 42-inch setback.

But it is all academic from here. I bought the new jack, and it is rated at twice the load as the old one. Now here is an anecdote to go with the story:

The jack separated into two parts while towing on the interstate highway in Northern Michigan. I heard a clunk and looked in the rear view mirror on the passenger side in time to see something skitter across the road from the trailer to the shoulder. We pulled over immediately. I got out of the car to go look for whatever had come off the trailer. About 100-yards back I found half of the tongue jack. I was walking back with it along the highway when a Michigan State Trooper pulled over. He yelled over to me, "What do you think you're doing?"

I guess he thought I was out for a walk on the interstate highway shoulder. I replied, "Something came off my trailer and I went back to retrieve it."

This seemed to soften up the trooper's attitude. I walked back to my truck and trailer and he followed with his blue light rack flashing. When we got back to the truck and trailer, the trooper got out to inspect the scene.

It was clear that the trailer jack part was from my trailer. The trooper then said, "That is a small jack for such a big boat."

"Well," I said, "it has been working for 20 years. I guess something vibrated loose."

I tossed the jack part into the back of the truck. Then the trooper turned to me and said, "Hey, how do you like that E-TEC engine?"

We then had a short chat on the side of the road about E-TEC engines. The trooper was thinking about re-powering his boat. I think he might get an E-TEC.

Next I got back in the truck and with the trooper's help got back on the highway and up to speed. We had pulled off at the base of a long grade and it took us a while to accelerate to 55-MPH going uphill from a dead start. Eventually the trooper pulled ahead and took off.

About ten miles down the road we saw him again, this time off the road and hiding so as to run RADAR on the speeders. When we went by he gave us a wave from his hiding post in the trees.

End of trailer jack story.

Robert V posted 08-20-2012 03:22 PM ET (US)     Profile for Robert V  Send Email to Robert V     
Oh boy, I thought the "Tongue Weight" discussion was tough! Just wait, now the word "E-TEC" has been introduced to the thread! Let me get this tangent started, "I just heard Mercury has introduced a revolutionary, super-turbo charged, ultra-light weight, maintenance free........". All in good fun,

Robert

Newtauk1 posted 08-20-2012 03:51 PM ET (US)     Profile for Newtauk1    
I suggest the author spend more time maintaining his trailer properly then pondering physics questions. The roads would be a lot safer if he did. He is lucky his neglect did not injure someone.
JBCornwell posted 08-20-2012 07:22 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Though the mathematical gymnastics are interesting, I don't see the point.

Reminds me of the myth about NASA spending megabux trying to invent a ballpoint that would write in zero gravity when the Russians simply used a pencil.

Why not weigh the tongue?

Red sky at night...
JB

jimh posted 08-20-2012 09:36 PM ET (US)     Profile for jimh  Send Email to jimh     
I wish I were as prescient as Newtauk1 who appears to be able to predict the future. Some are gifted like him, I am not. I just used 9th-Grade Physics to compute a load. The jack had worked perfectly at its last use, with the jack in compression. I don't know anyone who tests their jack in tension, nor have I ever read of any recommendation to test a jack in tension instead of compression. Perhaps our new soothsayer can explain the method.

JB misses the point. I have already weighed the tongue. I simply computed the load on the jack.

I am sorry that 9th Grade Physics gets so many readers riled up. I thought perhaps by calculating the load on the jack that others might realize that the load on the jack is considerably greater than the tongue weight. This might be of use to someone.

JBCornwell posted 08-21-2012 07:15 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Okay. I see the point now. :)


Red sky at night. . .
JB

Newtauk1 posted 08-21-2012 11:23 AM ET (US)     Profile for Newtauk1    
[Changed topic to begin a discussion of the discussion of the discussion itself and other participants. I can't say this enough, apparently, but CONTINUOUSWAVE is not interested in collecting, organizing, publishing, and archiving this user's opinion about the website, about its articles, and about the participants. We collect, organize, and publish articles about boating. --jimh]
jimh posted 08-21-2012 12:20 PM ET (US)     Profile for jimh  Send Email to jimh     
I am closing this thread. I was satisfied with the mathematics and physics after the first reply. Apparently math and physics are hostile topics for some boaters, but they should just relax and go read other articles which contain no math and physics.

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